Great Duck Island Storm Petrels

Leach’s Storm Petrel

Data source: Gnam and Sheridan 2023.

Study Site

Methods

Data

Variables of Interest

Side of island

Variables of Interest

Side of island
Distance to Ocean

Histogram

Histogram (nonzero counts)

In case you were curious…

linearModel <- lm(Total~Dist+Side, data = all)
plot(linearModel)

Are the data overdispersed?

Poisson distribution assumes:

\[ {\frac{variance}{mean}} \approx 1 \]

Data (with 0s):

var(all$Total)/mean(all$Total)

[1] 9.869822

Zero-inflated GLM

Two different models

Can poisson error be met without 0s?

Data (without 0s):

var(nonzero$Total)/mean(nonzero$Total)

[1] 6.423672

Data (without 0s, log):

var(nonzero$Ln)/mean(nonzero$Ln)

[1] 0.650921

var(nonzero$Log10)/mean(nonzero$Log10)

[1] 0.2826914

Verbal Model - Poisson ANVOCA

Does distance to the coast and side of the island impact petrel burrow counts?

Model Variables
Source	DF	Fctr/Cov	R/F
Side	1	Fctr	fixed
Dist	1	Cov	fixed
SidexDist	1x1		ff

Response variable: count of burrows in quadrat

ANOVA

anova <- aov(Total~Dist+Side+Dist*Side, data = all)
summary(anova)

             Df Sum Sq Mean Sq F value Pr(>F)    
Dist          1     13    13.2   0.488  0.485    
Side          1   2941  2940.9 108.267 <2e-16 ***
Dist:Side     1      2     1.6   0.059  0.809    
Residuals   604  16407    27.2                   
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

Poisson ANCOVA for non-zero data

\[ Total = e^\eta + \varepsilon \] \[ \eta = \beta_0 + \beta_dX_d +\beta_sX_s + \beta_{d x s}X_dX_s \]

Where: d = distance to ocean s = East/West side of island Poisson error distribution

Poisson ANCOVA model in R

poissonModel <- glm(Total~Dist+Side+Dist*Side, data = nonzero, family = poisson(link = log))
plot(poissonModel)

Poisson ANCOVA model in R


Call:
glm(formula = Total ~ Dist + Side + Dist * Side, family = poisson(link = log), 
    data = nonzero)

Coefficients:
             Estimate Std. Error z value Pr(>|z|)    
(Intercept)  1.729294   0.119517  14.469  < 2e-16 ***
Dist        -0.004789   0.001156  -4.143 3.43e-05 ***
SideW        0.483828   0.129603   3.733 0.000189 ***
Dist:SideW   0.002607   0.001273   2.048 0.040546 *  
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

(Dispersion parameter for poisson family taken to be 1)

    Null deviance: 1565.0  on 293  degrees of freedom
Residual deviance: 1387.1  on 290  degrees of freedom
AIC: 2380.2

Number of Fisher Scoring iterations: 5

Maybe a Gamma distribution?

gammaModel <- glm(Total~Dist+Side+Dist*Side, data = nonzero, family = Gamma())
plot(gammaModel)

Maybe a Gamma distribution?


Call:
glm(formula = Total ~ Dist + Side + Dist * Side, family = Gamma(), 
    data = nonzero)

Coefficients:
              Estimate Std. Error t value Pr(>|t|)    
(Intercept)  0.1638703  0.0479744   3.416 0.000727 ***
Dist         0.0013250  0.0005319   2.491 0.013296 *  
SideW       -0.0559233  0.0503825  -1.110 0.267930    
Dist:SideW  -0.0010386  0.0005586  -1.859 0.063996 .  
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

(Dispersion parameter for Gamma family taken to be 0.7872139)

    Null deviance: 262.46  on 293  degrees of freedom
Residual deviance: 230.06  on 290  degrees of freedom
AIC: 1664.4

Number of Fisher Scoring iterations: 6

Will proceed with poisson.

Verbal Model - Neg. Binomial GzLM

Does distance to the coast and side of the island impact presence of petrel burrows?

Model Variables
Source	DF	Fctr/Cov	R/F
Side	1	Fctr	fixed
Dist	1	Cov	fixed
SidexDist	1x1		ff

Response variable: presence of burrows in quadrat

ANOVA Table

anova <- aov(Present~Dist+Side+Dist*Side, data = all)
summary(anova)

             Df Sum Sq Mean Sq F value Pr(>F)    
Dist          1   1.23    1.23   6.302 0.0123 *  
Side          1  32.46   32.46 166.078 <2e-16 ***
Dist:Side     1   0.10    0.10   0.516 0.4730    
Residuals   604 118.05    0.20                   
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

Negative Binomial GLM

\[ Odds = e^\eta \]

\[ \eta = \beta_0 + \beta_dX_d +\beta_sX_s + \beta_{d x s}X_dX_s \]

Where: d = distance to ocean s = East/West side of island Error distribution is binomial/ Bernoulli

Neg. Binoimial GLM in R

presentabsent <- glm(Present~Dist+Side+Dist*Side, family = binomial(link = logit), data = all)
plot(presentabsent)

Neg. Binoimial GLM in R

library(statmod)
res <- qresid(presentabsent)
qqnorm(res)
qqline(res)

Neg. Binoimial GLM in R

summary(presentabsent)


Call:
glm(formula = Present ~ Dist + Side + Dist * Side, family = binomial(link = logit), 
    data = all)

Coefficients:
             Estimate Std. Error z value Pr(>|z|)    
(Intercept) -1.635830   0.278333  -5.877 4.17e-09 ***
Dist         0.005048   0.002443   2.067   0.0388 *  
SideW        1.918985   0.357368   5.370 7.88e-08 ***
Dist:SideW   0.001773   0.003495   0.507   0.6120    
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

(Dispersion parameter for binomial family taken to be 1)

    Null deviance: 842.21  on 607  degrees of freedom
Residual deviance: 699.34  on 604  degrees of freedom
AIC: 707.34

Number of Fisher Scoring iterations: 4

The Zero Inflated GLzM

library(pscl)
zeroInflated <- zeroinfl(Total ~ Dist+Side+Dist*Side |
                 Dist+Side+Dist*Side,
               dist = 'poisson',
               data = all)
summary(zeroInflated)


Call:
zeroinfl(formula = Total ~ Dist + Side + Dist * Side | Dist + Side + 
    Dist * Side, data = all, dist = "poisson")

Pearson residuals:
    Min      1Q  Median      3Q     Max 
-1.5947 -0.8068 -0.4596  0.1611  8.0925 

Count model coefficients (poisson with log link):
             Estimate Std. Error z value Pr(>|z|)    
(Intercept)  1.749518   0.123315  14.187  < 2e-16 ***
Dist        -0.005510   0.001254  -4.394 1.11e-05 ***
SideW        0.464602   0.133155   3.489 0.000484 ***
Dist:SideW   0.003306   0.001363   2.425 0.015309 *  

Zero-inflation model coefficients (binomial with logit link):
              Estimate Std. Error z value Pr(>|z|)    
(Intercept)  1.6771608  0.2841251   5.903 3.57e-09 ***
Dist        -0.0062192  0.0025813  -2.409    0.016 *  
SideW       -1.9568037  0.3621482  -5.403 6.54e-08 ***
Dist:SideW  -0.0006878  0.0036027  -0.191    0.849    
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1 

Number of iterations in BFGS optimization: 12 
Log-likelihood: -1532 on 8 Df

E2 <- resid(zeroInflated, type = "pearson")
N  <- nrow(all)
p  <- length(coef(zeroInflated)) + 1 # '+1' is due to theta
sum(E2^2) / (N - p)

[1] 2.058631

Likelihood Ratio for Side

*Excluding interaction term, since the coefficient is very small.

withoutSide <-  zeroinfl(Total ~ Dist | Dist, dist = 'poisson', data = all)
withSide <-  zeroinfl(Total ~ Dist + Side| Dist + Side, dist = 'poisson', data = all)

library(lmtest)
lrtest (withoutSide, withSide)

Likelihood ratio test

Model 1: Total ~ Dist | Dist
Model 2: Total ~ Dist + Side | Dist + Side
  #Df  LogLik Df  Chisq Pr(>Chisq)    
1   4 -1669.9                         
2   6 -1535.4  2 269.01  < 2.2e-16 ***
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

LR for Distance from Ocean

withoutSide <-  zeroinfl(Total ~ Side | Side, dist = 'poisson', data = all)
withSide <-  zeroinfl(Total ~ Dist + Side| Dist + Side, dist = 'poisson', data = all)

library(lmtest)
lrtest (withoutSide, withSide)

Likelihood ratio test

Model 1: Total ~ Side | Side
Model 2: Total ~ Dist + Side | Dist + Side
  #Df  LogLik Df  Chisq Pr(>Chisq)    
1   4 -1556.8                         
2   6 -1535.4  2 42.972  4.663e-10 ***
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

Is distance to ocean ecologically meaningful?

Bonus: Occupancy Rate

Occupancy Rate

Do the number of active burrows scale with the number of total burrows?

Model?

occupancy <- glm(Active~Total, family = gaussian(link = identity), data = nonzero)
plot(occupancy)

Coefficients


Call:
glm(formula = Active ~ Total, family = gaussian(link = identity), 
    data = nonzero)

Coefficients:
            Estimate Std. Error t value Pr(>|t|)    
(Intercept)  -0.7882     0.1319  -5.977 6.62e-09 ***
Total         0.5968     0.0141  42.321  < 2e-16 ***
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

(Dispersion parameter for gaussian family taken to be 2.501663)

    Null deviance: 5211.16  on 293  degrees of freedom
Residual deviance:  730.49  on 292  degrees of freedom
AIC: 1107.9

Number of Fisher Scoring iterations: 2

Occupancy

Questions?